Discontinuous transmission, generally referred to as DTX, is a general designation of all concepts where a temporary break in payload data to be transmitted causes a corresponding break in an otherwise continuous or regular stream of scheduled transmissions. The most typical example is the application of DTX to a telephone connection. As a first approximation, the participant of a point-to-point telephone call is only speaking for one half of the time, because during the remaining time he is silent and listening to the speaker at the other end. If a battery-driven mobile telephone is adapted to only produce full-scale transmissions when its user is actually speaking, transmission capacity in the telephone network can be saved and battery life extended considerably.
However, it is not advantageous to mute all radio transmissions even during periods when payload data is not available for transmission. For purposes like link quality estimation, channel estimate updating, transmission power control and synchronisation it is advantageous to maintain a thinned-out schedule of brief transmissions in the absence of payload data. Also a speech codec that is not receiving actual speech data should nevertheless regularly receive so-called silence descriptors that it uses for updating the spectrum of artificially generated background noise, also known as comfort noise.
Concerning conventional speech-based DTX arrangements we may make a simplified statement according to which it was on the responsibility of the speech codecs to generate the necessary silence time transmissions: during an observed break in speech a transmitting speech codec modelled the continuous background noise and used it to generate the silence descriptors. The so-called Layer One (L1) mechanisms, which are responsible for low-level radio interface functionalities like channel estimation and receiver synchronisation, could rely on the silence descriptor transmissions coming often enough to be used also for the other purposes. For example an AMR (Adaptive MultiRate) speech codec standardised for the GSM (Global System for Mobile telecommunications) cellular radio system transmits a silence descriptor once in every 160 milliseconds.
However, the advent of 3GPP (Third Generation Partnership Project) has changed the picture. The roles of the RAN (Radio Access Network) and the CN (Core Network) are now more clearly separated, so that the RAN is only supposed to offer the transport channel for whatever service there may exist between a mobile station and a core network. Different kinds of core networks may utilise the same RAN for communicating with mobile stations, and from a certain core network there may be connections to mobile stations through different kinds of RANs. The radio interface between the RAN and a mobile station may be completely identical regardless of whether the Iu interface on the other side of the RAN operates with a packet-switched or a circuit-switched core network.
According to the 3GPP approach, the RAN does not necessarily even know, what kind of services go through the “transport channel tubes” maintained in the RAN. The lack of such knowledge in the RAN has necessitated defining certain functions that the RAN is supposed to apply independently, in order to support L1 functionalities such as synchronisation management and link quality estimation.
The standard 3GPP TS 45.008, where TS comes from Technical Specification, requires dummy blocks belonging to L2 (layer two) to be sent over the radio interface to satisfy the needs of L1 functionalities during silent periods, if PDTCH or FLO is in use. Of these, PDTCH means a Packet Data Traffic Channel and FLO means Flexible Layer One, which is a way of redefining certain L1 functionalities in a parameterised way so that their optimisation for specific purposes can be made case by case through choices made by higher levels in the OSI (Open System Interconnection) model. However, regardless of any of PDTCH or FLO being used to carry a speech service, considerations related to the operation of the speech codecs require the transmission of silence descriptors independently of said dummy blocks. The result may be a situation where, during a break in the transmission of payload data, both dummy blocks and silence descriptors are transmitted. Their transmission moments might coincide in time in an ideal case, but since their generation processes are independent of each other, such temporal coincidence would be unlikely. A major part of the advantages of DTX could be lost, because these two partly redundant processes might easily produce an excessive number of silent-time transmissions.